The present invention relates generally to telecommunication systems and more particularly to an interface device for providing analog telephone functionality in connection with a digital telephone network.
In a traditional analog telephone system, each telephone or other communication device ("subscriber unit") is typically interconnected by a pair of wires ("tip" and "ring" wires or, cooperatively, "subscriber lines," "subscriber loop" or "phone lines") through a series of equipment to a switch at a local telephone company office ("central office"). At the central office, the tip and ring lines are interconnected to a device known as a "subscriber line interface circuit" or "SLIC," which provides required functionality to the subscriber unit. The switches at the central offices are interconnected to provide a network of switches thereby providing communications between, e.g., a local subscriber and a remote subscriber.
The SLIC is an essential part of the network's interface to individual analog subscriber units. The functions served by the SLIC include providing talking battery, ring voltage, ring trip, off-hook detection, and call progress signals such as ringback, busy, and dial tone. In many business office environments, the small-scale function of a central office is assumed by a PBX system, which, in turn, may include a number of SLICs to provide interconnected subscriber units with the required functionality.
Analog subscriber units generate and receive analog signals. While most modern telephone networks are digital and route digital signals from point to point, the subscriber units are still predominantly analog. This is possible because the analog signal generated by an analog subscriber unit is converted to digital form for transmission through the network, and is converted at the remote location back to analog form for transmission over the subscriber loop for reception by a remote analog subscriber unit. The analog signals generated and received by analog subscriber units generally take the form of voice frequency signals for end-to-end communications between local and remote subscriber units. Such signals generally represent human speech, or may be modulated signals which are treated by the phone network as if they were ordinary speech signals.
Other analog signals generated or received at the analog subscriber units are supervisory signals which are not intended for transmission to a remote terminal. Rather, they are designed to communicate with the network to enable functions such as call initiation, call progress indication, and call termination. These signals include those provided by, or through, a SLIC such as (i) "talk battery voltage" which provides power to the analog subscriber unit; (ii) "ring voltage" which is a relatively high voltage indicative of an incoming call, (iii) call progress tones such as dial tone, busy tone, and ringback tone. These various signals and tones will be described below.
The subscriber line interface circuit provides DC power, or "talk battery" power, along the phone lines to enable operation of circuitry in subscriber units connected to those lines. All telephone systems work on DC (direct current) power. Typically, the talking battery voltage on analog phone lines is between 5 (off-hook) and 48 volts (on-hook). Most telephone systems and PBXs are connected directly to an AC (alternating current) outlet on the wall and convert that AC power to the DC power required by the phone system. Telephone company central offices are also often driven by rechargeable lead acid batteries, which not only provide necessary power in the event of a power outage but also serve as a filter to smooth out fluctuations in the commercial power and remove the "noise" that power often carries. Talking battery must provide sufficient voltage to enable analog telephones to perform functions such as amplification and sound pickup as well as other modern phone functions such as DTMF keypads and speakerphone operation. The talking battery power supply should always be available to an analog telephone, in the event the phone is placed in an off-hook, or closed-circuit state.
A subscriber line interface circuit provides a ring voltage signal to the analog subscriber unit to cause the analog phone to ring in the event of an incoming telephone call. Analog phone systems recognize a ring voltage signal placed on the phone lines by the SLIC, and in turn generate an audible electronic or mechanical ring sound to alert the subscriber of an incoming call. In order to ensure that an analog phone will recognize the ring voltage signal, the ring voltage is required to be 70 to 90 volts (or 140 to 180 volts peak-to-peak AC) at a frequency of 17 to 20 Hz. Significantly, voltages of this magnitude may be harmful if accidentally applied to circuitry intended for operation at a lower voltage. Therefore, in the event of a malfunction, the SLIC is required to meet certain safety specifications and requirements relating to electrical isolation.
For example, European Standard EN 60950, entitled Particular Safety Requirements For Equipment To Be Connected To Telecommunication Networks, defines a hazardous voltage as a voltage exceeding 42.4 volts AC (peak) or 60 volts DC. Any components within a circuit that are connected to voltages above these limits must be electrically isolated (and in some cases, physically isolated) from the lower voltage components that are typically in connection with central office circuits. Components requiring isolation include the power supply and the tip and ring connections on the customer/subscriber side of the telephone network.
The power supply isolation requirement is typically satisfied by providing an additional secondary winding within the voltage transformer of the DC-to-DC converter which is used to generate the high amplitude ring voltage. The line isolation is typically satisfied by a line transformer. In general, however, these isolation requirements impose additional costs associated with the added componentry.
A SLIC also passes call progress tones such as dial tone, busy tone, and ringback tone to the subscriber unit For the convenience of the subscriber who is initiating the call, these tones are provided by the central office as an indication of call status. When the calling subscriber lifts the handset, or when the subscriber unit otherwise generates an "off hook" condition, the central office generates a dial tone and supplies it to the calling subscriber unit to indicate the availability of phone service. After the calling subscriber has dialed a phone number of the remote (answering) subscriber unit, the SLIC passes a ring back sound directed to the calling subscriber to indicate that the network is taking action to signal the remote subscriber, i.e., that the remote subscriber is being rung. Alteratively, if the network determines that the remote subscriber unit is engaged in another call (or is already off-hook), the network generates a busy tone directed to the calling subscriber unit.
The SLIC also acts to identify the status to, or interpret signals generated by, the analog subscriber unit. For example, the SLIC provides -48 volts on the ring line, and 0 volts on the tip line, to the subscriber unit. The analog subscriber unit provides an open circuit when in the on-hook condition. In a "loop start" circuit, the analog subscriber unit generates an off-hook condition by providing a termination, i.e., by closing, or "looping" the tip and ring to form a complete electrical circuit. This off-hook condition is detected by the SLIC (whereupon a dial tone is provided to the subscriber). Most residential circuits are configured as loop start circuits. Some countries, however, have other requirements. Germany, for example, requires a ground to be applied on an additional lead that acts as a control signal.
The SLIC must also be able to detect the off-hook condition during application of ring voltage. That is, when a call is incoming and the SLIC is providing the high amplitude ring voltage signal to the analog subscriber unit, the SLIC must be able to detect when the analog subscriber unit goes off-hook to answer the call. This is known as "ring trip." The SLIC must immediately cease the ring voltage signal upon detection of the off-hook condition, and provide the analog subscriber unit with the voice channel signals originating from the distant end subscriber unit.
The SLIC must also pass Dual Tone Multi-Frequency (DTMF) signals generated by the analog subscriber unit to the network. This signaling format is a well known method of providing dialing information. Each number on a keypad array is represented by two separate tones, one tone identifying the column, and the other representing the row. Together, two tones uniquely identify a digit. These tones are passed along to the network by the SLIC.
A digital transmission system such as integrated services digital network ("ISDN") provides digital information all the way to the subscriber unit. The central office uses special digital interface equipment on subscriber loops that provides ISDN services. The system also requires the presence of digital subscriber units to interpret this digital information. Although the central office equipment and the subscriber equipment is digital in nature, this digital information is typically transmitted along the same copper wires used by plain ordinary telephone service (POTS) from the central office to the customer premises. Basic ISDN service is known as 2B+D, for two Bearer (B) or subscriber channels, and one Data (D) channel for control and signaling data. The B channels are 64 K bits per second, and the D channel is typically 16 K bits per second. These data rates are significantly faster than rates of traditional analog modems that can provide data rates in the neighborhood of 28.8 K bits per second or 33.6 K bits per second.
A typical ISDN configuration is shown in FIG. 1. The two-wire interface between the ISDN switch 10, and the customer premises is known as a "U" interface 12. An ISDN Network Terminator ("NT1") 14 converts this two-wire interface to a four wire "S/T" interface 16. The modulation format used between the central office and the NT1 is known as 2B1Q in North America, and 4B3T in Europe and Japan. The ISDN NT1 14 in turn demodulates the signal and passes the resulting digital information along the S/T bus 16 to a digital subscriber unit, also known as Terminal Equipment 1 (TE1) such as an ISDN phone 18, or alternatively, to an ISDN modem 20 for providing a communications link between computer devices.
There are many such ISDN modems commercially available which allow computers to connect to other computers or to networks of computers, via ISDN communication lines. Analog devices are designated as TE2, 22, and operate on what has been designated an R interface 24. To operate on the S/T bus 16, a TE2 analog device requires a terminal adapter (TA) 26. Typically, a TA might utilize a SLIC integrated circuit to provide the necessary interface functionality.
In the United States, customers are typically provided with a U interface by the service provider, and customers must provide and maintain their own NT1 termination for interconnecting devices designed for S/T interfaces. Of course, for convenience, an ISDN modem designed for use in the United States may also include the circuitry necessary for connection directly to an ISDN U interface. One disadvantage of a device having a built-in NT1 interface is that it might prevent other devices from being connected to the S/T bus if the device does not provide external access to the S/T bus.
It is well understood that analog subscriber units of the type described above will not function in an ISDN environment without appropriate TA interface circuitry. Indeed, devices exist that provide an analog interface for ISDN communications links. There are TA's that provide RJ-11 telephone connection plugs and RS-232 interfaces. The present invention provides an improved interface with numerous advantages over the prior art devices.